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100 W-class Circlotron

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I have lately been testing with a circlotron monoblock with two JJ EL509. My goal has been to design about 100 W class amplifier with these tubes. The present circuit is based on my earlier prototype I built couple years ago. Then it was a test platform for different output tubes and transformers. At that time I made some test with 6L6 and EL34 but now wanted to go to higer power level. I added an adjustable screen grid voltage to accommodate my circuit for EL509 that requires relatively low G2 voltage.

The whole project has been delayed because I did not have suitable power supplies but finally – after I had burned a dozen of power NFET’s – I managed to get ready my adjustable 600 V / 300 mA power supply. The other supply is two series connected Heathkit IP-2717 which are a bit light, but will do.

The required 100 W could be achieved with some 450 V supply voltage and 500 ohms load impedance, but then the linearity of the output tubes is not optimal. Better alternative is to use higher load impedance (1k) and higher supply voltage (550 V).
It follows that the driving voltage level at the output tubes is very high (> 500 Vpp) and this in turn places high demands on the linearity and capacity of the driver stage, but this challenge can be solved.

The output transformer I used is quite unusual. It is Polish Indel TGL40/001 that is originally 4k to 8 ohms type. This transformer contains four separate parallel connected secondaries and by connecting two in series and two in parallel the required 1k to 8 ohms output impedance was created.
One good feature of this transformer is it’s low ( 1,4 mH) leakage inductance and this can be clearly seen. The frequency response exceeds above 40 kHz without any GNFB.
Usually the high end frequency response is problematic with circlotrons.

Below is the present circuit:

An externally hosted image should be here but it was not working when we last tested it.


I made some tests before and after applying 10 dB global negative feedback.

Without GNFB I got as follows:

+Ub = 550 V, Ik = 55 mA/tube, Ug2 = 120 V, Ug1 = -37 V, Rload = 8 ohms

Pout(max.) = 120 W @ THD = 2,0 %

THD at: 1 W = 0,10 %, at 30 W = 0.27 %, at 60 W = 0,33 % and at 100 W = 1,2 %

Frequency response (-1 dB) <20 Hz….40 kHz

Zout = 1,8 ohms

With 10 dB GNFB :

THD at 1 W = 0,05 %, at 50 W = 0,11 %, at 110 W = 0,39 % and at 120 W = 0,53 %

Frequency response (-1 dB) <20 Hz….70 kHz

Zout = 0,16 ohms

At 30 Hz and 110 W, THD = 0,70 %
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Looks very nice.Some remarks however.
I hope each EL509 has it own heatersupply ,Vkf=150V max !,connected to its cathode.
The poor 6N1 has a hard time with Va=300V max (Svetlana says only 250V)
What is the use of R65 ?
How come you get 350V after the two 33k ? If one 500V goes up the other goes down the same amount and v.versa.Netto from 500V with two resistances in parallel to 350V ,how about 2x 100k.Or do I misunderstand ?
First unit completed

I finally got one of the monoblocs done and one more is under process.

I built the power supply and amplifier on separate boards. Then possible repair and modifications are easier to accomplish.
There are a current meter for both EL509 to monitor the standing current if the bias tends to fluctuate. With full 100 W output power to 8 ohms load the anode current of one EL509 is 150 mA.
At idle it is some 55 mA.

The test results are a little better now when I adjusted the GNFB to 12 dB. For example there is a THD sweet spot at 70 W; only 0,08 %. At 100 W THD is essentially below 0.5 %.
Un-weighted hum level is –90 dB and A-weigted below –100 dB.

I will do a complete test for both amplifiers as soon as I get the 2nd unit ready.

There will be no volume control, but a switchable –10 dB / 0 dB input attenuator.

An externally hosted image should be here but it was not working when we last tested it.

An externally hosted image should be here but it was not working when we last tested it.
I got 10 amplifier and 5 power supply pcbs for 110 USD including shipping from iTead studio.
When the pcb size is 100 mm x 100 mm the unit price is very low. Therefore I try to fit my designs to this size if possible. I did not make the pcb for just two amplifiers.

I will update the schematics soon.
Your monobloc looks great and those pcbs make it look more professional, nice job!
If I were you I'll try to place final tubes a little bit distanced itself from each other because they get really hot.
Can you measure its TID (total intermodulation distortion) ? Also what is your Indel transformer primary inductance? and secondary?
The reason for output tube placement is in the PCB pricing.
The size 10 cm x 10 cm is essentially cheaper than for example 13 cm x 7 cm.

10 pcs. of 10 cm x 10 cm amplifier PCB's cost 25 USD, but 5 pcs. of 13 cm x 14 cm power supply PCB's cost 70 USD.

The tube heat is not a problem. The free space between tubes is 25 mm and the cooling seems more than sufficient.

There are few different ways to measure the IMD. What specs do you mean ?
In general, if the THD is good then the linearity is good and IMD must be good too.

The primary inductance is 5H and at secondary it is obviously 8 ohms/1k x 5H = 40 mH.
Thanks for the primary inductance value, I want to simulate that schematic sometime.
One more question what current flows through 6N1P triodes?
Your observation is correct, usually if thd is good, tid must be good too. I was pointing to the 2 tones IMD test having 9kHz and 10kHz equal amplitudes at the input and looking to 1kHz and 19kHz harmonics value compared to the fundamentals. (for this measurement a custom wave generator or 2 sine wave generators are needed)
About placing final tubes... you can wire the socket to the pcb and this way you maintain a smaller pcb area and a good spacing between final tubes. I've worked with EL509 russian version and they are a little larger than JJ version and they get quite hot.
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